Screened and Shielded Cabling - Noise Immunity, Grounding, and the Antenna Myth
Design of Screens and Shields
Shielding offers the benefits of significantly improved pair-to-pair crosstalk performance, alien crosstalk performance, and noise immunity that cannot be matched by any other cabling design strategy. Category 6A and lower rated F/UTP cables are constructed with an overall foil surrounding four twisted-pairs as shown in figure 5.
FIGURE 5: F/UTP Construction
Category 7 and higher rated S/FTP cables are constructed with an overall braid surrounding four individually foil shielded pairs as shown in figure 6. Optional drain wires are sometimes provided.
FIGURE 6: S/FTP Construction
Shielding materials are selected for their ability to maximize immunity to electric field disturbance by their capability to reflect the incoming wave, their absorption properties, and their ability to provide a low impedance signal path. As a rule, more conductive shielding materials yield greater amounts of incoming signal reflection. Solid aluminum foil is the preferred shielding media for telecommunications cabling because it provides 100% coverage against high frequency (i.e. greater than 100 MHz) leakage, as well as low electrical resistance when properly connected to ground. The thickness of the foil shield is influenced by the skin effect of the interfering noise currents. Skin effect is the phenomenon where the depth of penetration of the noise current decreases as frequency increases. Typical foil thicknesses are 1.5 mils (0.038mm) to 2.0 mils (0.051mm) to match the maximum penetration depth of a 30 MHz signal. This design approach ensures that higher frequency signals will not be able to pass through the foil shield. Lower frequency signals will not interfere with the twisted-pairs as a result of their good balance performance. Braids and drain wires add strength to cable assemblies and further decrease the end-to-end electrical resistance of the shield when the cabling system is properly connected to ground.